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- Muscles burn glucose during the first 5 minutes of exercise and glycogen during long exercise periods (from 5-15 minutes). - Exercise longer than 15 minutes burns fatty acid fragments stored in muscles and muscle protein. - We burn fat after we are finished exercising. - All cells use glucose as t...

- Muscles burn glucose during the first 5 minutes of exercise and glycogen during long exercise periods (from 5-15 minutes). - Exercise longer than 15 minutes burns fatty acid fragments stored in muscles and muscle protein. - We burn fat after we are finished exercising. - All cells use glucose as the first energy source, so carbohydrates need to be a part of a daily diet. - There are 10 calories per gram of lipid and 4 calories per gram of carbohydrates, so lipid metabolism produces more energy, but takes longer. - Fat is burned during times of muscle rest - All cells need lipids for plasma membranes, so it needs to be a part of a daily diet. - **Protein Metabolism** - The body contains more than 20,000 different proteins assembled from the same 20 amino acids. - Constant turnover of proteins in cells, so protein is vital to our metabolism. - Weight loss of 4-5 pounds per week is safe and usually will not allow for muscle wasting (digestion of proteins from muscles if calorie intake is too low for demand). - About 4 calories per gram of protein, the same as for carbohydrates. - Metabolism of proteins produces urea, which is toxic to us, so extracted in liquid urine. - 8 amino acids cannot be metabolized in the body and so must be consumed daily, therefore called essential amino acids - Vegetarians & vegans must eat enough essential amino acids to remain healthy. - Diets with inadequate amounts of protein can lead to protein deficiency diseases such as Kwashiorkor and Marasmus which have the characteristic swollen bellies and sunken faces. - Newborns in the US are tested for PKU, a disease in which the body cannot produce the enzyme to convert *phenylalanine* to tyrosine. - So phenylalanine accumulates in the brain causing brain damage; so these patients eat a diet low or lacking in phenylalanine, and they develop normally.